In recent years, from a global environmental conservation standpoint, there has been a strong demand for improved vehicle fuel efficiency. Thus, high-strength steel sheets are actively utilized as automotive body components in order to reduce the weight of automobile bodies. High-strength steel sheets are utilized not only in automobile structural parts but also in underbody parts and truck frame parts. In general, higher-strength steel sheets have lower workability. In particular, because automotive parts are formed by intense processing, steel sheets used as automotive body component materials are strongly required to have high strength and good workability.
To address such demands, for example, Patent Literature 1 describes a high-strength hot-rolled steel sheet having a composition containing, on a mass percent basis, C: 0.05% to 0.15%, Si: 1.50% or less, Mn: 0.5% to 2.5%, P: 0.035% or less, S: 0.01% or less, Al: 0.020% to 0.15%, and Ti: 0.05% to 0.2%, wherein the high-strength hot-rolled steel sheet has a microstructure containing 60% to 95% by volume bainite and solute-strengthened or precipitation-strengthened ferrite or ferrite and martensite and has good hole expansion formability, which is shown by a fracture transition temperature of 0° C. or less in a Charpy impact test. In a technique described in Patent Literature 1, after hot rolling, a steel sheet is cooled to a temperature in the range of 400° C. to 550° C. at an average cooling rate of 30° C./s or more, is coiled, and is cooled to a temperature of 300° C. or less at a cooling rate in the range of 50° C./h to 400° C./h. Patent Literature 1 states that such a process can prevent diffusion of P in grain boundaries, decrease the fracture transition temperature to 0° C. or less, improve toughness, and improve hole expansion formability.
Among automotive body components, truck frame parts and underbody parts are particularly frequently bored in order to join parts or reduce their weight or in order for a subsequent burring process or bore expanding process. In general, such boring is performed by punching from the perspective of productivity. Thus, improved punchability is often strongly required.
However, the technique described in Patent Literature 1 only prevents the intergranular segregation of P and improves hole expansion formability. Patent Literature 1 does not describe stamping workability. It is difficult to believe that the prevention of segregation of P in grain boundaries can directly improve punched surface properties and contribute to improved stamping workability.
With respect to improvement of stamping workability, for example, Patent Literature 2 proposes a high-strength hot-rolled steel sheet having a composition containing, on a mass percent basis, C: 0.01% to 0.07%, N: 0.005% or less, S: 0.005% or less, Ti: 0.03% to 0.2%, and B: 0.0002% to 0.002%, having a microstructure containing ferrite or bainitic ferrite as a main phase and a hard second phase and cementite, the hard second phase and cementite having an area fraction of 3% or less, and having good stamping workability. It is stated that a technique described in Patent Literature 2 can retain B in a solid solution state and thereby prevent defects in punched surfaces. In the technologies described in Patent Literature 2, ferrite or bainitic ferrite is the largest area phase, and the hard second phase, which adversely affects hole expandability, is limited to 3% or less.
Patent Literature 3 proposes a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and good punchability. The high-strength hot-rolled steel sheet has a composition containing, on a mass percent basis, C: 0.05% to 0.15%, Si: 0.1% to 1.5%, Mn: 1% to 2%, P: 0.03% or less, S: 0.003% or less, Al: 0.01% to 0.08%, Ti: 0.05% to 0.15%, and N: 0.005% or less and has a bainite phase having an area fraction of more than 95%. The average particle size of the bainite microstructure at a quarter thickness in the depth direction is 5 m or less in a vertical cross section parallel to the rolling direction and 4 μm or less in a vertical cross section perpendicular to the rolling direction. A region having a thickness of 1/10 of the sheet thickness at a central position in the thickness direction contains 7 or less crystal grains having an aspect ratio of 5 or more each spreading in the rolling direction. The technologies described in Patent Literature 3 is intended to improve punchability by decreasing the average particle size of bainite and by decreasing the number of spreading grains in a central region in the thickness direction.